1. Field of the Invention
The present invention relates to a document feeder, and more particularly to a document feeder which picks up one document from a stack of documents and feeds the document at a constant speed over a stationary image reading device.
2. Description of Related Art
As disclosed by U.S. Pat. No. 5,461,468, a type of document feeder which reads an image of a document with a raster input scanner which is parked at a position while feeding the document on a reading position over the parked input scanner at a constant speed is well known. This is called a feed-scan reading type. In the document feeder disclosed by U.S. Pat. No. 5,461,468, a nudger roller which comes down for feeding of a document picks up some documents from a stack on a tray, and a first feed roller and a retard roller separate one from the other documents. Then, take-away rollers and registration rollers feed the single document toward the reading position correcting possible skew of the document, and a plurality of CVT (constant velocity transport) rollers transport the document on the reading position at a constant speed. In order to minimize the interdocument gap, the nudger roller, the first feed roller and the take-away rollers are driven at a comparatively high speed, and then, when the leading edge of the document reaches the registration rollers, the speed is lowered. The registration rollers and the CVT rollers are driven at the same speed.
In this feed-scan reading type document feeder, skew of a document is corrected by the registration rollers which are located upstream of the first CVT rollers which are located right before the reading position. Therefore, skew of a document which occurs after the registration rollers cannot be corrected, and the document may be transported askew on the reading position. Also, the fact that the registration rollers are away from the reading position makes it impossible to deskew the next document during image reading of the previous document, and this limits minimization of the interdocument gap. Further, if during image reading of a document, the next document comes into the nip portion of the registration rollers, the load increases suddenly, which influences the constancy of the transport speed. Consequently, the reproductivity of the document image deteriorates.
Further, in the feed-scan reading type document feeder, in handling duplex documents which have images on respective both sides, in order to keep the page order of the documents, each document must be fed through the reading position three times. More specifically, a document is circulated three times for inversion and then discharged onto a discharge tray, and thereafter, the next document is picked up and fed to the reading position. However, in this type, the efficiency of image reading is bad, and the copy productivity is low.
Recently, copying machines are operable in an APS mode wherein a copy sheet size is automatically selected in accordance with the document size, in an AMS mode wherein a copying magnification is automatically selected, in a four-in-one mode wherein four document images are copied on a single copy sheet, and/or in a two-in-one mode wherein two document images are copied on a single copy sheet. In operations in these modes, it is preferred that a document is picked up and fed toward the reading position and is detected by a document size detector provided in the transport path as early as possible. However, in the above-described way of handling duplex documents, feeding of the next document is delayed, and it is impossible to heighten the copy productivity efficiently in operations in the above modes.
Further, in using a digital image reader, the required capacity depends on the size and the number of documents, and if the capacity of the memory is too small to store the data of the documents, image reading must be interrupted. For judgment on the interruption, feeding of the next document and detection of the document size must be carried out as early as possible. Also, since digital image readers take more time for processing, early feeding of the next document is desired.